This application claims priority of European Patent Application EP01811217.7, the content of which is hereby incorporated.
The present invention concerns a measuring machine, notably a column for measuring longitudinal dimensions, for example a height-measuring column.
Height-measuring columns are described for example in document U.S. Pat. No. 4,924,598. They are used for example for measuring or comparing dimensions, for example in mechanical workshops. The measuring column represented diagrammatically in FIG. 1 comprises a fixed supporting frame 2 with a base 20, a carriage 3 that can be displaced vertically along the supporting frame, a device for driving the carriage and a system for measuring the carriage""s vertical position. A probe tip 44 is connected to the carriage and is designed for being brought into contact with the piece to be measured. The base can be provided with means for creating an air-cushion in order to easily displace the height-measuring column on the work surface.
The driving device can be hand-operated and actuated by means of a crank, or electrically operated using a motor. The crank or the motor drives a cable or a belt 40 connected to the carriage 3 as well as a counterweight 41 moving in opposite direction from the carriage. The cable or belt is tensed between pulleys 42, 43. The carriage 3 comprises wheels (not represented) for pressing on the guiding rails attached to the supporting frame.
The electronic measuring system allows the position of the carriage, and thus of the probe tip, to be determined and displayed on an electronic display. The resolution and precision that is expected of this type of measuring columns is on the order of the micron.
This precision depends for an important part on the contact force between the probe tip 44 and the piece to be measured. A substantial contact force causes a flexion of the probe tip and/or of the piece, or even an elastic deformation of the material, that can influence the measuring. The contact force between the probe tip and the piece to be measured must thus be minimal or, in any case, identical at each measuring.
Because of the constraints associated with the maximal admissible space requirement and with manufacturing precision, the belt 40 is not always strictly parallel to the supporting frame 2. In particular, it has been observed within the framework of this invention that the fastening angle xcex1 of the belt 40 onto the carriage 3 varies according to the carriage""s longitudinal position. This is notably the case when the carriage is close to either the lower pulley 43 or upper pulley 42 of the column 1 and when the radius r of the pulleys is different by the distance d between the supporting frame 2 and the fastening point 30 of the belt 40 on the carriage. It is also the case when the position of the center of the pulleys 42, 43 relative to the supporting frame is not controlled precisely. In these circumstances, the traction force of the belt on the carriage is exerted along a non-vertical direction and causes a moment m to be applied along a horizontal axis parallel to the plane of the carriage 3. This moment influences the contact force between the probe tip and the piece, so that the measuring precision varies according to the carriage""s longitudinal position.
U.S. Pat. No. 5,040,308 describes a measuring column in which the measuring carriage is connected to the upper end of the driving belt through an adjusting piece. The carriage can slide relative to the adjusting piece in the direction of the measuring axis. However, the articulation between the adjusting piece and the carriage allows only a displacement along the measuring axis. The components of the traction force perpendicular to the measuring axis are thus exerted on the carriage and indirectly on the probe tip.
Patent application EP223736 describes a measuring column in which the measuring carriage is connected to the upper part of the driving belt through a frame. The carriage can pivot and slide horizontally relative to the frame. No means has however been provided to allow the carriage to pivot relative to the frame along a horizontal axis parallel to the plane of the carriage and of the frame. Any moment exerted by the driving cable on the frame will thus invariably be reflected on the probe tip.
Furthermore, the measuring probe tip is placed along an axis y parallel to the plane of the carriage and of the frame. This construction is not very advantageous since the probe tip finds itself far away from the reference surfaces constituted by the guiding rails on the supporting frame. Furthermore, the probe tip is close to an edge of the supporting frame which could risk coming into contact with certain parts of a piece of complex shape.
It is thus an aim of the present invention to propose a height-measuring column in which these problems are solved.
In particular, it is an aim of the present invention to propose a height-measuring column in which the non-vertical components of the force exterted by the driving system on the carriage are reduced.
According to the invention, these aims are achieved by means of a measuring column comprising the characteristics of the independent claim.
In particular, these aims are achieved by means of a column for measuring longitudinal dimensions comprising a supporting frame, a carriage capable of moving along a measuring axis along the supporting frame, a probe tip connected to the carriage and designed to be brought into contact with the piece to be measured, a device for driving the carriage comprising a cable or belt for moving said carriage along said measuring axis, a system for measuring the position of said carriage along said measuring axis, and in which an articulation element between said cable or belt and said carriage allows a pivoting at least along a pivoting axis parallel to the carriage""s plane.
This construction has the advantage of applying on the carriage, and thus on the probe tip, only the vertical components of the traction force exerted by the cable or belt. It is thus possible to avoid measuring errors due to a horizontal component of this force, notably when the carriage finds itself close to one of the two pulleys.